1. Field of the Invention
The invention relates to a process for producing wood particleboard by pressing wood particles which have been treated with binder, in which the wood particles are boiled and milled at elevated temperature under steam pressure in a refiner unit, and subsequently fluidized in a stream of steam in a blow-line, then dried and pressed under pressure and, if desired, at elevated temperature, to produce boards.
2. Background Art
To produce wood particleboard, the wood particles, for example wood fibers or wood chips, are glued together by means of an organic adhesive under pressure and at elevated temperature. The most important binders for fiberboard production are urea-formaldehyde resins (UF resins). To produce moisture-resistant wood chipboards, phenol-formaldehyde resins (PF resins) are of great importance. Melamine-formaldehyde resins (MF resins) are also used for improving the moisture resistance of wood particleboard. A disadvantage of these adhesives is that formaldehyde is emitted both during production of the particleboard and during use of the finished, pressed particleboard. A further disadvantage of these reactive resins becomes apparent in the production of MD and HD fiberboard: in the production of medium density fiberboard (MDF) and high density fiberboard (HDF), the fibers are hydrothermally pretreated in a first step in a refiner unit, i.e. boiled and milled at elevated temperature under steam pressure. After milling, the fibers, while still under steam pressure and at temperatures from 120° C. to 150° C., are treated with binder by spraying an aqueous dispersion of the binder via a cooled valve into the blow-line. The turbulence which occurs at a flow velocity of from 200 to 500 m/s distributes the binder uniformly over the fiber surface. Finally, the fibers which are treated with binder are dried, laid down uniformly, and pressed at temperatures of from 150 to 250° C. to produce boards. A problem is that during the treatment with binder in this process, the reactive resins react in the blow-line as a result of the elevated temperature, resulting in a loss of up to 25% of their binding potential during pressing.
Formaldehyde-free, thermally curable, aqueous binders for producing wood particleboard are known, for example, from WO-A 97/31059. In this publication, a mixture of carboxyl-functional copolymer and an alkanolamine having at least two hydroxy groups is used. Aqueous adhesive compositions comprising polycarboxylic acid and hydroxyalkyl-substituted aminoaliphatics are described in WO-A 97/45461. WO-A 99/02591 relates to compositions comprising a carboxyl-functional copolymer and long-chain amines. A disadvantage of these systems, which crosslink via an esterification reaction, is that crosslinking occurs only in the water-free state, on drying.